The present invention relates to a method of making high carbon content steel engine components, and more particularly, it relates to a process of making high carbon content steel engine components using cold-forming techniques.
There are several known blanking processing techniques available for manufacturing engine components from steel. Specific examples of such techniques include screw machining, warm forming, and hot forging. Screw machining involves taking bar stock and machining the stock, using a multi-spindle machine, into a raw shape, or starting blank. The starting blank is then subjected to further machining to tolerance. While effective for generating a starting blank, material waste tends to be high for this operation.
Warm forming involves heating a supply wire below a critical temperature to improve malleability. For high carbon content steel (steel having a carbon content between 0.93-1.05%) such as SAE 52100 material specified in ASTM A295, the critical temperature is 1330xc2x0 F. Once heated, the material is formed into a desired shape. However, the warm forming technique is disadvantageous because the shaped parts cannot meet the tight tolerances required for many applications. More specifically, as the shaped part cools, it deforms, thereby requiring additional machining steps to achieve the desired shape.
Hot forging starts with either bar stock, supply wire or a slug. The starting material is heated in a manner similar to warm forming, but to a higher temperature. More specifically, the material is heated above the critical temperature but below its melting point. Once heated, the material is hammered into the desired shape. However, similar to warm forming, as the shaped part cools, the part deforms. Moreover, the tooling used to perform the hammering operation tends to be crude and imprecise such that additional machining is required to achieve the desired shape.
After the shaped part is formed, the part is then heat treated. After heat treating, the shaped part is then subjected to multiple grinding operations; end grind, outside diameter grind, inside diameter grind and outside diameter finishing operations. Once the part is complete it is fitted with mating components and assembled into final assembly.
In addition to machining, warm forming and hot forging, it is known to employ cold forming techniques for produce complex parts to tight tolerances, or xe2x80x9cnear net shapexe2x80x9d. For example, it has been known to employ cold-forming techniques, such as upsetting, heading, and extrusion for the manufacture of high strength nuts and bolts. However, such cold forming techniques have only been proven effective for forming complex parts using low carbon content material to avoid tool damage and cracking of the part during manufacturing.
However, complex parts that experience high contact stress, such as cam follower rollers, must be produced from high carbon content steel (e.g. 52100 grade). Previous attempts to cold form high carbon content steel have resulted in several problems. One problem experienced was that the near net shape components tended to crack due to work hardening of the supply material. Another problem experienced was increased wear on the forming tools, and in many instances failure of the tools.
To alleviate the difficulty in cold forming high carbon content steel, specific processing steps have been employed to process the high carbon content steel prior to cold forming. Traditionally, processing of high carbon content steel wire has included multiple drawing operations to yield the desired physical properties. Because material volume is critical during the forming process to assure adequate die fill, steel supply wire processing has included a two step drawing process, with the first draw resulting in a major cross sectional area reduction of 25% or greater. The second drawing operation is a more precisely controlled sizing draw with a 5% or less area reduction.
An example for standard processing for supply wire is as follows. First, a standard wire rod with a starting diameter of 18 mm is provided. The starting wire is then annealed. A zinc-phosphate coating is then applied to act as a lubricant and the wire is drawn to 15.5 mm (25.8% reduction). The drawn wire is annealed again. A peeling operation is then performed, peeling the diameter down to 14.7 mm. The wire is then wire brushed and drawn to 14.5 mm (a 2.7% reduction). The wire then undergoes an eddy current check to check for defects and a zinc-phosphate coating is applied over the wire to act as a lubricant through further processing.
While somewhat effective, as the number of operations required to process the wire increase, so do costs. Thus, there exists a need for a cost effective method of manufacturing engine components of high carbon content steel (i.e., having a carbon content greater than approximately 0.65%) the includes a method of processing high carbon content steel that is adequately formable to substantially limit damage to forming tools. Moreover, there exists a need for a cost effective method of manufacturing engine components of high carbon content steel that permits displacement of material from a center of the blank, to conserve material and reduce costs.
The present invention relates to a cold forming manufacturing process to produce precision, hardened and ground engine components from high carbon steel (carbon content greater than 0.70%). To avoid the deficiencies in the prior art, namely limited tooling life and cracks in the final finished part, the first step in the process includes providing a preprocessed high carbon content steel starting material that has increased formability and reduced internal material stress.
In accordance with the present invention, a high carbon content steel supply wire having a carbon content greater than at least 0.70%, and even more preferably, greater than 0.90%, is specifically processed. The first step of processing the supply wire includes annealing to increase its formability, and then coating the supply wire with a lubricant, such as zinc phosphate. Next, the annealed supply wire is drawn to a first predetermined diameter, such that the supply wire undergoes at least a 25% reduction. After the drawing operation, the drawn supply wire is peeled to a second predetermined diameter to remove surface defects. The supply wire is then wire brushed.
The brushed supply wire is next subjected to a second drawing operation. The second drawing operation is more controlled and reduces the diameter by less than 5%. After drawing, the supply wire is eddy current checked for defects. According to one aspect of the invention,.the drawn supply wire is annealed again to provide increased formability during cold forming. Finally, the annealed supply wire is coated with a lubricant, such as zinc phosphate or an organic material.
Once preprocessed, the supply wire is next cold formed into a xe2x80x9cnear net shapexe2x80x9d meaning that the part is produced substantially close to final dimensions. The cold forming operation is performed through either heading or extrusion. Due to the use of cold forming, the supply material is forced out into the near net shape, thus minimizing waste and reducing or eliminating grinding operations. In accordance with the present invention, the cold forming operation is performed in multiple operations and in progressive steps to avoid work hardening, thereby avoiding cracking and damaging to both the part being formed and the forming tools. Moreover, because the part is formed to near net shape, no additional machining is required, only minimal grinding to hold certain features of the part to very tight tolerances and to improve surface finish.